The answer is Assam.
Mizoram is a state in northeastern India. It is bordered by Myanmar to the east, Bangladesh to the south, Tripura to the southwest, Assam to the west, and Nagaland to the north. The capital of Mizoram is Aizawl.
Nagaland is a state in northeastern India. It is bordered by Assam to the west, Manipur to the south, and Myanmar to the east and north. The capital of Nagaland is Kohima.
Assam is a state in northeastern India. It is bordered by Bhutan to the north, Arunachal Pradesh to the northeast, Nagaland to the east, Manipur to the southeast, Mizoram to the south, Tripura to the southwest, Meghalaya to the west, and West Bengal to the northwest. The capital of Assam is Dispur.
Tripura is a state in northeastern India. It is bordered by Bangladesh to the south, Mizoram to the west, Assam to the north, and Meghalaya to the east. The capital of Tripura is Agartala.
Assam is the only state among the four that lies to the west of Manipur.